Thermal transfer type printers are known which form (or print) an image on a recording medium (paper sheet) by pressure-contacting a thermal head against a peripheral face of a platen roller with interposition of the recording medium and an ink ribbon. In the thermal transfer type printer, an ink ribbon cassette is set removably in which an ink ribbon of a predetermined width is wound around a cylindrical ribbon cores. In the ink ribbon cassette, a ribbon-delivery core holds an unused ink ribbon wound around it and delivers the ink ribbon, and another core, a ribbon-winding-up core, winds up the used ink ribbon.
In the image formation by the printer, a platen roller is rotated at a constant rotation speed to deliver the ink ribbon at a constant delivery speed from the ink ribbon-delivery core. The ribbon-delivery core, while holding a sufficient amount of the unused ink ribbon wound thereon, is rotated at a low rotation speed since the outside diameter of the wound ribbon is large. With progress of delivery of the ink ribbon from the ribbon-delivery core, the outside diameter of the wound ribbon gradually decreases to increase gradually the rotation speed of the ribbon-delivery core. In contrast, on the ribbon-winding-up core, the outside diameter of the wound ribbon increases with progress of the delivery of the ribbon from the ribbon-delivery core, resulting in gradual decrease of the rotation speed thereof.
Accordingly, the amount of the remaining unused ink ribbon can be estimated by measurement of the rotation speed of the ribbon-delivery core (or the ribbon-winding-up core) to estimate the outside diameter of the wound ribbon. For measurement of the rotation speed, in a known technique, a flange having a magnetic member is provided at a lengthwise end of the ribbon core, and the rotation speed is measured by detecting the magnetic force lines generated by the magnetic member (e.g., Japanese Patent Laid-Open No. 2003-211801)